In the field of transmission systems for work vehicles, such as agricultural tractors and construction vehicles, various transmission configurations and control schemes have been proposed and are in use. Such transmissions typically include a collection of intermeshing gears either fixed to transmission shafts or rotating freely on the shafts. Clutches associated with the freely rotating gears may be selectively engaged to establish a series of speed ratios between an engine output shaft and a transmission output shaft to transmit engine torque at a desired speed to driven wheels of the vehicle. Control systems for commanding engagement of the clutches typically include electronic circuitry that responds to operator controls, such as an upshift/downshift lever, a forward/reverse lever and the like in the vehicle cab. The control system sends electric signals to hydraulic valves that channel pressurized fluid to the clutches. The control systems cause the clutches to engage and disengage in predetermined combinations to accelerate, decelerate and drive the vehicle as desired by the operator. Transmissions and control systems of this type are described in U.S. Pat. No. 4,425,620, assigned to Steiger Tractor, Inc., and U.S. Pat. No. 4,967,385, assigned to J. I. Case Company.
The electronic systems for controlling transmissions of large, off-road work vehicles typically provide for various modes of operation and for control of a large number of forward and reverse gear ratios. The various operation modes and large number of gear ratios allow an operator to efficiently and accurately control the movement of these vehicles under a variety of differing load conditions without overloading the engine or clutches of the drivetrain, and without causing engine overspeed. The operator may use an upshift/downshift control lever to request a speed change via the selection of an appropriate gear ratio, wherein the selection occurs by control of solenoid valves which control engagement of hydraulic clutches corresponding to the appropriate gear ratio. In a certain system configured for operation in a forward manual mode, actuation of the control lever causes the system to upshift by a single gear ratio change to the next higher gear ratio. To allow upshifts by more than one gear ratio, the control lever can be held in its actuated position to cause the system to sequentially upshift by a single gear ratio at a time at a predetermined shift rate to obtain a desired gear ratio.
However, due to the large number of gear ratios provided by the transmissions of work vehicles, sequentially shifting through each intermediate gear ratio between a current gear ratio and a desired gear ratio can adversely affect vehicle operation. For example, since each upshift takes a certain length of time (e.g., 0.75 seconds), the rate of acceleration for such vehicles can be limited, and may be too low under conditions such as when the vehicles enter the stream of traffic on a road, or when the vehicles cross a highway. There are at least three situations wherein sequential shifting through each intermediate gear ratio can be disadvantageous: when the shift lever changes the commanded gear ratio faster than the shift rate of the transmission; during transmission engagement from neutral to a selected forward or reverse gear ratio; and during shuttle shifting operations.
Problems associated with sequentially shifting through each intermediate gear ratio can be particularly troublesome during shuttle shifting, wherein the transmission is continually shifted between a desired forward gear ratio and a desired reverse gear ratio. In one work vehicle equipped with a forward-neutral-reverse-park (FNRP) selection lever and a gear ratio "bump" lever for incrementing and decrementing the commanded gear ratio, the bump lever is used to preselect forward and reverse gear ratios to which the transmission will be shifted during shuttle shifting, while the FNRP lever is used to continually shift between the preselected forward gear ratio and the preselected reverse gear ratio.
A solution to these problems is known as "skip shifting", wherein the transmission control system skips some intermediate gear ratios between the current and the desired gear ratios. For example, the transmission control system shown in U.S. Pat. No. 5,233,525, commonly assigned to Case Corporation and incorporated herein by reference, includes an upshift/downshift control operated by a pulser lever to provide upshift and downshift pulses to an electronic controller. A skip shift control including a button is located at the distal end of the pulser lever. Actuation of the skip shift button by the vehicle operator causes an electrical signal to be sent to the controller. In response, the controller selects an alternative gear selection control program which alters the sequence of engaged gear ratios from continuous sequential selection of all gear ratios to only a portion thereof. By skipping certain gear ratios from the sequence of engaged gear ratios, performance is enhanced.
Although the above-described control system advantageously provides for skip shifting of a power transmission to enhance vehicle acceleration, the system requires actions by the operator before skip shifting may be invoked. In particular, before skip shifting occurs, the operator must determine that skip shifting would be desirable for the particular conditions faced by the vehicle, and must also depress the skip-shift button accordingly. These actions must occur at the same time that the operator is performing a variety of other actions, such as watching for traffic and steering the vehicle as the vehicle enters the stream of traffic on a road or crosses a highway. Due to other tasks facing the operator, the skip-shift button may not be actuated at the correct time, or may not be actuated at all, thereby obviating the benefits of having the skip shifting function in the first place.
Other transmission systems provide a shift "bump" switch located on the gear shift lever. Moving the gear shift lever causes the controller to upshift or downshift the transmission into the respective adjacent gear ratio. Actuating the shift bump switch while the gear shift lever is being moved causes the controller to bump the gear ratio up or down every other gear ratio to move through the gear ratios faster. As with the previously-described system, however, this system puts the burden on the operator to actuate the shift bump switch at the appropriate times. Also, bumping the transmission by a constant number of gear ratios may adversely affect shift quality under certain conditions depending on the current gear ratio of the transmission, the load on the vehicle, and the engine/transmission combination. For example, skip upshifting by a constant number of gear ratios under certain conditions may put too large an instantaneous load on the engine, and may cause the engine to lug down. Conversely, skip downshifting by a constant number of gear ratios under other conditions may decrease the instantaneous load on the engine too quickly, and may cause an engine overspeed and possible engine damage.
Thus, it would be advantageous to provide a transmission control system capable of performing automatic skip shifts without contemporaneous input from the operator. It would also be advantageous if such a control system could provide for skip shifting by a variable number of gear ratios depending upon the current gear ratio of the transmission to provide acceptable shift quality throughout the entire range of gear ratios. It would be advantageous to provide a transmission control system which provides for automatic and variable skip shifting when the transmission is in one of certain predetermined gears, while disabling skip shifting in other predetermined gears. In addition, it would be advantageous to provide a control system wherein such skip shifting is implemented in a straight-forward manner easily adaptable for different vehicle applications. Further, it would be advantageous to provide a transmission control system wherein direct engagement of certain gear ratios is allowed, and skip shifting is provided for other ratios.